The present invention relates to the magnet arts. It finds particular application in conjunction with magnetic resonance imaging apparatus having self-shielded gradient coil assemblies and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with magnetic resonance spectroscopy systems and other applications in which a polarizing magnetic field.
Magnetic resonance imagers commonly include a bore having a diameter of 90 cm or more for receiving the body of an examined patient. The bore is surrounded by a series of annular superconducting magnets for creating a substantially uniform magnetic field longitudinally through the patient receiving bore. The more axially spaced the annular magnets, the more uniform the primary magnetic field within the patient receiving bore tends to be and the longer the axial dimension over which such magnetic field uniformity exists. Typically, the length of the bore is at least 1.75 times the diameter and often twice the diameter or more. Such "long" bore magnets are designed such that there are substantially no measurable degradation from spherical harmonics below twelfth order, commonly denoted Z12.
Within the specified imaging region of a magnetic resonance imaging (MRI) magnet, the ideal magnetic field is one which is perfectly uniform. The specified imaging region is normally defined by a sphere of a specified diameter, typically 50 cm.
The bare magnet field is defined as the state of the magnetic field measured at a specified set of coordinates lying on the surface of a spherical volume when the superconducting coils of the magnet are energized, but without the presence of any additional correcting fields from whatever source. This bare magnet field is, in the ideal case, perfectly uniform by virtue of the design of the magnet. However, because of inevitable manufacturing inaccuracy, the field will actually have a finite range known as the inhomogeneity of the bare magnet. The inhomogeneity is usually defined as the range in the field expressed in parts per million of the central field. Thus, the lower this number is, the better is the field quality from an imaging point of view.
Shimming is a technique for correcting the bare magnet field for its non-uniformity. The bare magnet field can be pictured as a large constant field with a small inhomogeneous field components superimposed on that constant field. If the negative of the inhomogeneous components of the field can be generated, then the net field will be made uniform and the magnet is said to be shimmed.
Active shimming is a commonly-used method of shimming using a set of coils surrounding the spherical volume. These coils are adjusted to carry DC currents which generate magnetic fields designed to cancel out the non-uniformities of the magnet.
Passive shimming, on the other hand, uses a magnetic field arising from the induced magnetic dipole of pieces of ferromagnetic steel placed inside the magnet bore in shim trays. The shim trays are removably mounted around the magnetic bore. The prior art shim trays each extend the length of the bore and have 12-14 pockets for receiving plates of iron or steel. The shim trays are removed and plates of iron or steel are mounted in the pockets until the magnetic field sensors indicate that the Z1-Z6 harmonics have been minimized, i.e., components whose inhomogeneity behaves as Z.sup.1 through Z.sup.6. Typically, magnetic resonance magnets are designed without higher order components. If higher order spherical harmonics are present, they require more steel to effect a correction than do lower order harmonics. Typically, shimming for higher order harmonics also requires substantially higher positional accuracy.
The idea is to use the dipole field of the steel to compensate for the inhomogeneous component of the bare magnet field. Introducing a single piece of steel into the magnet (but outside the spherical volume), the dipole field from the steel modifies the net field throughout the spherical volume. However, in general it is unlikely that a single piece of steel would be sufficient to compensate fully for an irregularly shaped inhomogeneous field. Therefore, in general, a large array of similar pieces of steel are added at locations surrounding the spherical volume. The individual dipole fields from all the steel shim pieces is superposed. If this net superposition of dipoles cancels the original inhomogeneous field, then the field is substantially uniform.
The steel is added to shim set locations within the shim trays called pockets. This is performed in an iterative process that works well for the shimming of low order spherical harmonic components (e.g., Z1-Z6). A major disadvantage of previous techniques is that once steel is added, the shimming technique does not allow for the possibility of removing any of that steel in subsequent iterations. This leads to the use of more steel than may be optimal and also the use of more shim pocket positions than may be necessary.
The prior shimming techniques have only been successful for shimming low order components (less than or equal to Z6). Generally, these techniques are not used to shim out high order harmonic components (greater than Z6).
One of the disadvantages of long bore magnets is that the region of interest is often inaccessible to medical personnel. If a procedure is to be performed based on the image, the patient must be removed from the bore before the procedure can be performed. Moving the patient increases the risk of misregistration problems between the image and the patient. Other disadvantages of the long bore magnets is that they tend to be user-unfriendly and claustrophobic, larger magnets are more expensive than smaller magnets, and the like.
One way to improve access to the patient is to shorten the length of the magnet and the patient receiving bore. If the magnet and the bore were shortened, patient access would be much improved. Although the size of the uniform magnetic field compresses from a sphere toward a more disk-like shape, the area of substantial uniformity is still sufficient for a series of 10-20 contiguous slice images.
However, shortening the bore of the magnet has its difficulties. The magnetic field tends to become yet more non-uniform. The inventors herein have measured significant higher order harmonic distortions in such short bore magnets. The Z12 harmonic is found to be relatively strong.
In accordance with the present invention, a magnetic resonance magnet is provided which is shimmed for improved uniformity through at least the Z18 harmonic.